The invention relates to a bimetal controlled snap disc thermal switch of the type disclosed, for example, in German Patent Application No. 2,530,971.
Such a switch is responsive to the temperature of a heating plate, for example a surface cooking unit in a household kitchen range, and, upon reaching a certain maximum desired temperature, acts on the heating current in such a manner as to prevent further increases in the temperature of the plate. This can be done by directly interrupting the heating current by means of a switch element or reducing the heating current, or otherwise adapting that current to the desired temperature.
The bimetal snap disc in such switches is arranged so as to be in thermal contact with a support plate which itself is arranged so as to be suspended below the heating plate or a heat transfer plate of the device.
For operationally reliable performance of the bimetal snap disc it is of significance, on the one hand, that the curvature of the snap disc is able to snap over from a convex curvature at rest through the dead point, or snap point, into a concave operating curvature; on the other hand, however, the disc should remain in constant thermal contact with the support plate since only then is it assured that the bimetal curvature will react immediately with respect to the temperature of the heating plate.
It is of further significance that the first curving movement of the snap disc, when the heating plate is heated up will not be transferred directly to the electrical switch element, actuated by the snap disc by means of a transfer member, since this would result in a reduction of the switch contact pressure and possibly to disadvantageous burning of the switch contacts due to a creeping, or gradual, contact opening. For this reason, it is known to make the bimetal snap disc act on the switch element only after a certain dead time, i.e. to provide a certain amount of play in the transfer mechanism between bimetal disc and switch element. To assure now that the bimetal disc will rest in good thermal contact against the support plate in every operating position, it is also already known to transfer to the bimetal disc, via the transfer mechanism, a spring bias directed toward the base plate, and compression springs are provided for this purpose in the region of the switch.